1. Field of the Invention
This invention relates to an intake system for an engine, and more particularly to an intake system for an engine having a primary intake passage which is opened during operation of the engine under light load and a secondary intake passage which is opened during operation of the engine under heavy load.
2. Description of the Prior Art
In the intake system of an engine disclosed in Japanese Unexamined Utility Model Publication No. 61(1986)-57166, there are provided primary and secondary intake passages for each of the cylinders, the primary intake passage being opened during a light load operation of the engine and being curved in order to form swirl of intake air in the combustion chamber and the secondary intake passage being opened during a heavy load operation of the engine. In the intake system, when the engine is operating under light load, intake air is introduced into the combustion chamber only through the primary intake passage, thereby forming swirl of intake air so that fuel is better mixed with air, and when the engine is operating under heavy load, intake air is introduced into the combustion chamber through both the primary and secondary intake passages, thereby ensuring sufficient amount of intake air.
However the intake resistance through a curved intake, passage is inherently higher than that through a straight intake passage. Accordingly, when the primary intake passage is curved in order to form swirl of intake air during a light load operation of the engine, the amount of intake air is apt to be short during a heavy load operation of the engine due to high intake resistance through the curved primary intake passage even if both the primary and secondary intake passages are opened.